We launched the Body Weight & Puberty Study (BWPS; www.bodyweightandpuberty.niehs.nih.gov) the first pediatric-focused study ever to be conducted in the NIEHS Clinical Research Unit (CRU), in December 2015. We have enrolled over 70 subjects. In this protocol, pre-menarchal girls between the ages of 8 and 14 undergo a physical exam (Tanner staging of breast and pubic hair; height, weight, and waist-hip ratio), breast ultrasound (morphological staging of glandular development), pelvic ultrasound (ovarian and uterine volumes, follicle counts, endometrial thickness), DEXA (% body fat), hand x-ray (bone age), and blood and urine tests (sex steroids, vaginal maturation index VMI of urogenital epithelial cells) Visit 1. Study procedures are repeated in 6 months Visit 2. Preliminary data from BWPS demonstrates that: 1) obese girls staged as pubertal on physical exam (Tanner stage > 2) tend to have immature breast morphology on ultrasound relative to their normal weight counterparts, and 2) breast maturation scores correlate with other end-organ effects of estrogen (eg ovarian size, uterine size, bone age, and VMI) to the same degree in overweight/obese and normal weight girls, suggesting that thelarche in overweight/obese girls is driven by activation of the central components of the reproductive axis and hence ovarian estradiol, as opposed to peripheral sources of estradiol(eg adipose tissue).